The cost and energy associated with the production and/or mining of raw materials for use in consumer and industrial products, together with the cost, energy, and environmental consequences of disposal the byproducts formed during fabrication of finished products, and the disposal of finished products themselves post-use, creates enormous incentives towards finding secondary uses for unwanted materials. Enormous investments in recycling and waste to energy technologies over the past several decades are testament to the economic advantages inherent in re-using and recycling materials. Competitive pressures, combined with these economic advantages, have resulted in the adaptation of waste minimization technologies, recycling, and re-use programs by virtually all major manufacturing industries. However, despite the economic advantages created by these strategies, massive volumes of the byproducts of industrial production and finished products post-use are still typically in a condition unsuitable for re-use and recycling, and huge volumes of these materials are disposed of in landfills and hazardous waste disposal sites on a daily basis.
Some of the more vexing difficulties that prevents more widespread adaptation of re-use and recycling strategies are related to the inherent hazardous nature of many of these materials, the cost and expense of processing heterogeneous streams of these waste materials, and finding applications for re-use that would consume significant volumes of these materials. One application for re-use that has long been recognized as having the potential for high volume usage is as a construction material. For example, concrete is exceeded only by water in the commercial tonnage used annually in the United States. Thus, any product or bi-product that could conceivably be utilized as an aggregate in a concrete mixture would have an enormous “sink” through which the materials could be re-used and thereby incorporated into valuable products. These advantages have led to the incorporation of fly ash generated in commercial boilers and blast furnace slag into concrete for construction. Some of the advantages of using fly ash in concrete were set forth in the Naval Facilities Engineering Service Center (NFESC) Technical Report TR-2195-SHR “Alkali-Silica Reaction Mitigation State-Of-The-Art” by L. J. Malvar, published in October 2001, the entire contents of which are incorporated herein by this reference. As described in the NFESC report, the use of Class F (low calcium) fly ash as a replacement for Portland cement in amounts around 25% has been shown to mitigate the effects of the Alkali-Silica Reaction (ASR) in concrete. Briefly, the ASR takes place when silica is surrounded by high pH cement paste, typically as a result of the high calcium and other alkaline content of the cement. The silica in this environment tends to react with the calcium to form a gel of calcium silicate which tends to swell over a period of time, typically ranging from as little as a few months to a few years. This swelling causes stress in the concrete, thereby weakening it. As further pointed out in the NFESC report, the 25% Class F fly ash replacement also resulted in less expansion than 35% Class C fly ash, which the authors note had previously been shown to either not reduce, or to aggravate the ASR problem. This is because the Class C fly ash exhibits a larger percentage of calcium. Thus, among the drawbacks associated with the widespread re-use of fly ash as an additive to concrete are differences associated with the differing types of fly ash, and their effect on the ASR in the final product.
Problems associated with the use of waste materials in concrete and ASR have also received notable attention with respect to glass materials. In U.S. Pat. No. 6,500,254, entitled “Cements including lithium glass compositions” issued Dec. 30, 2002 to Baxter et al., the entire contents of which are hereby incorporated herein by this reference, the inventors describe the use of lithium glass as an additive to concrete, and the minimization of ASR thereby. As described by the '254 patent, the lithium glass includes a glass forming oxide; lithium oxide; and optionally a glass network modifying oxide. The inventors further point out that the lithium glass can be essentially free of sodium or potassium ions. While the '254 patent does provide a means by which glass can be manufactured to be a useful additive to concrete, the use of the technology still has many practical drawbacks. For example, among the problems associated with the use of lithium glass as an additive to concrete is the energy intensive process through which glass is manufactured. As described by the inventors of the '254 patent, and as is also common in any commercial glass making operation, to form a glass requires the materials to be heated to a temperature sufficient to melt the glass, thereby allowing its formation. The high energy requirements for this melting operation often will dominate the economic benefits afforded by using glass as an additive. Further, the specific gravity of the resultant glass is similar to that of normal gravel aggregate. Thus, the resulting concrete produces similar or worse weight to strength ratios found in more traditional concrete mixtures.
Improved strength to weight ratios are the subject of wide interest in the concrete industry, and the search for suitable materials has attracted significant research and development investments. For example, Columbia-University has reported that “lightweight concrete is of utmost importance to the construction industry. Most of current research focuses on high-performance concrete by which is meant a cost-effective material that satisfies demanding performance requirements, including durability. The advantages of lightweight concrete are its reduced mass and improved thermal and sound insulation properties, while maintaining adequate strength. The reduced weight has numerous advantages, not the least of them being a reduced demand on energy during construction.” The concrete materials research group at Columbia University further reports that they have developed a “new kind of lightweight concrete, which combines the advantages of normal-weight aggregate with cellular concrete, that is, good strength and durability properties as well as thermal and sound insulation.” As described by the Columbia researchers, the key is “an admixture that introduces air bubbles into the cement matrix using normal mixing procedures and therefore can be combined with both normal and lightweight aggregate.” Thus, while the group at Columbia has apparently discovered a method for producing lightweight concrete, they have not addressed the need for the discovery of lightweight aggregates that incorporate byproducts or waste products in an economically advantageous way, or which would incorporate those materials in a manner which would prevent ASR.
Traditional approaches for large sized lightweight aggregate have included the use of materials such as perlite, expanded shale, and other naturally occurring porous rocks. Recently, research has examined sintered materials for use as a lightweight aggregate. Typically under either of these approaches, the material has an open structure, allowing it to absorb water. This can complicate the concrete forming process as the aggregate will often compete with the cement for available water. Thus, the aggregate is either soaked in water prior mixing, which can lead to excess water and less than optimal curing, or the aggregate is mixed with the cement immediately prior to use, thus necessitating the additional cost and inconvenience of mixing at or near a jobsite. Further, few small-sized lightweight aggregates have been developed. The industry still relies mainly on sand for small sized aggregates, which, while providing excellent flowability in the concrete mix prior to curing does not provide a particularly advantageous strength to weight ratio in the final product.
Therefore, there exists a need for methods and materials whereby byproducts or waste products can be incorporated into concrete mixtures in an economically advantageous way, which does not result in an unacceptable ASR. There further exists a need for methods and materials that will produce aggregate for concrete in a lightweight form, thereby allowing the formation of a final concrete product having a favorable strength to weight ratio. Most preferably, there exists a need for methods and materials that allow both the incorporation of industrial byproducts and waste products into concrete mixtures in an economically advantageous way in a lightweight form, thereby allowing the formation of a final concrete product having a favorable strength to weight ratio, and which does not result in an unacceptable ASR in the final concrete product. There also exists a need for lightweight aggregate materials that do not absorb water and which can be provided as having small particle sizes to allow a replacement for sand.